1. Field of the Invention
The present invention relates to a method and an apparatus for crystallizing a semiconductor.
2. Description of the Related Art
A liquid crystal display device includes an active matrix drive circuit which includes TFTs. Also, a system liquid crystal display device includes an electronic circuit including TFTs in the peripheral regions around the display region. Low-temperature polysilicon is suitable for forming TFTs for the liquid crystal display device and TFTs for the peripheral region of the system liquid crystal display device. Also, low-temperature polysilicon is applied to pixel driving TFTs for an organic EL display, an electronic circuit for the peripheral region of the organic EL display and the like. The present invention relates to a method and an apparatus for crystallizing a semiconductor, using a CW laser (continuous wave laser), for producing TFTs with low-temperature polysilicon.
In order to form TFTs of the liquid crystal display device with low-temperature polysilicon, in the prior art, an amorphous silicon layer is formed on a glass substrate, and the amorphous silicon layer on the glass substrate is irradiated by an excimer pulse laser to crystallize the amorphous silicon. Recently, a crystallization method has been developed wherein the amorphous silicon layer on the glass substrate is irradiated by a CW solid laser to crystallize the amorphous silicon.
In crystallization of silicon by means of the excimer pulse laser, mobility is in the order of 150 to 300 (cm2/Vs) but, on the other band, in crystallization of silicon by means of the CW laser, mobility in the order of 400 to 600 (cm2/Vs) can be realized, this being particularly advantageous in forming TFTs for electronic circuits in the peripheral region of the system liquid crystal display device.
In crystallizing silicon, the silicon layer is scanned by a laser beam. In this case, the substrate having the silicon layer is mounted on a movable stage, and the scanning is performed while the silicon layer is moved with respect to the fixed laser beam. As shown in FIG. 19, in the excimer pulse laser scanning, scanning can be performed with a laser beam having, for example, a beam spot “X” of 27.5 mm×0.4 mm, and the area scan speed is 16.5 cm2/s when the beam width is 27.5 mm and the scan speed is 6 mm/s.
On the other hand, as shown in FIG. 20, in the CW solid laser scanning, scanning can be performed with a beam spot “Y” of, for example, 400 μm×20 μm, and when scanning is performed at a scan speed of 50 cm/s, an acceptable crystallization melt width is 150 μm and the area scan speed is 0.75 cm2/s. In this manner, crystallization by means of a CW solid laser, polysilicon of excellent quality can be obtained but there is the problem that the throughput is low. Also, it is possible to perform scanning at the scan speed of 2 m/s, in which case the area scan speed is 5 cm2/s. However, the mobility of the polysilicon thus attained is low.
In crystallization by means of a CW solid laser, because the output of a stable CW laser is relatively low, even if the scan speed is increased, there is the problem that the area scan speed is low and throughput does not increase sufficiently.
In addition, if scanning is performed by the CW laser with a laser power of, for example, 10 W, the width “Y” of a beam spot of approximately 400 μm, and a scan speed of 50 cm/s, an effective melt width with a beam spot of 400 μm, at which acceptable crystallization can be attained, would be 150 μm, therefore the area scan speed is 0.75 cm2/s. In this manner, in crystallization by means of a CW solid laser, although polysilicon of excellent quality can be attained, there is still the problem of low throughput.
Further, as shown in FIG. 29, in the prior art, the movable stage supporting the substrate having the silicon layer comprises a Y-axis stage 1, an X-axis stage 2, a rotatable stage 3, and a vacuum chuck 4. Usually, the Y-axis stage 1, which is in the lowermost position, has a large high-speed structure that is highly mobile, and the X-axis stage 2, which is positioned above the Y-axis stage 1, has a relatively small and less mobile structure. The Y-axis stage 1 which is in the lowermost position takes the load of all of the upper components. A substrate including an amorphous semiconductor is secured the vacuum chuck 4, a laser beam is irradiated onto the amorphous semiconductor while the movable stages are moved, and the amorphous semiconductor is crystallized by being molten and hardened to form polysilicon.
With the excimer pulse laser, because the beam spot formed is relatively large, a high area scan speed can be achieved. However, with a CW solid laser, because the beam spot formed is extremely small, the area scan speed is quite low. Therefore, crystallization by means of a CW solid laser can achieve excellent quality polysilicon, but has low throughput.
In order to improve the throughput of crystallization by means of laser scanning, the substrate having the silicon layer must be moved reciprocally at the highest possible speed. In other words, the substrate is accelerated from a stationary state, continues to move at a constant speed while being scanned with laser, and thereafter is decelerated to a stationary state. Then, the substrate is moved in the opposite direction, at which time the substrate is accelerated, moves at a constant speed, and is decelerated to a stationary state. Laser scanning is executed while this reciprocal movement of the substrate is repeated.
In order to effectively perform high speed scanning, it is necessary to increase the acceleration/deceleration of the high speed Y-axis stage 1. However, if the acceleration is increased, the shock of acceleration is increased, and this shock is in proportion to the product of the acceleration and the weight of the loads supported by the stage. A large shock will vibrate the optical system for emitting the laser beam, shifting the adjustment thereof, thus putting the optical system out of focus and moving the focusing position, making stable crystallization unattainable.
In the prior art, because the Y-axis stage 1 which moves at high speed supports the load of all the other stage components, and the weight of this load is large, the acceleration thereof cannot be sufficiently increased and the substrate cannot be accelerated to a high speed in a short time.
Further, the rotatable stage 3 is used to correct dislocation of the rotation position of the substrate having the silicon layer, and can be rotated within the range of approximately 10 degrees. In order to rotate the substrate having the silicon layer 90 degrees, it is necessary to remove the substrate from the vacuum chuck 4 and reattach the substrate to the vacuum chuck 4. Consequently, in the prior art, 90 degree rotation of the substrate having the silicon layer is not performed.